The present invention relates to a downhole tool for providing a thrust force to an elongate body extending in a borehole formed in an earth formation. Such elongate body can be, for example, in the form of a drilling assembly used to drill the borehole. In conventional drilling operations a compressive load is exerted from surface and transmitted through the drillstring to the drill bit in order to generate sufficient compressive load on the bit, which compressive load is generally referred to as Weight On Bit. When the drilling assembly includes a relatively small diameter tubing which is unreeled at surface and lowered into the borehole as drilling proceeds, which tubing is also referred to as coiled tubing, the amount of compression which can be transmitted by such small diameter tubing is limited due to the risk of helical buckling and subsequent lock-up of the string.
Furthermore, if the borehole includes a horizontal section, a compressive load exerted to the drill string at surface will mainly result in the drill string being laterally pressed against the borehole wall in the horizontal section. Therefore, in the absence of measures taken to overcome these problems, the maximum available Weight On Bit during coiled tubing drilling is unacceptably limited, and horizontal borehole sections can only be drilled to a short length.
International patent application WO 93/24728 discloses a downhole tool for providing a thrust force to an elongate body extending in a borehole formed in an earth formation, the tool comprising at least one rotatable body provided with a plurality of rollers, each roller being expandable against the borehole wall at a selected contact force between the roller and the borehole wall, the rollers being oriented when expanded against the borehole wall so as to roll along a helical path on the borehole wall, and a motor to rotate each rotatable body.
When the rollers of the known tool are expanded against the borehole wall and the motor rotates the rotatable body, the tool has a tendency to move the elongate body forward through the borehole due to the helical path followed by the rollers. By the tendency to move forward the tool exerts a thrust force to the elongate body, which thrust force corresponds to the resistance encountered by the elongate body. When the thrust force is relatively high due to a high resistance of the elongate body, the rollers will slip along the borehole wall in circumferential direction thereof. It will be appreciated that by continued slippage of the rollers, the borehole wall becomes increasingly worn out so that the borehole diameter increases. Since the amount of radial expansion of the rollers is limited, continued slippage of the rollers leads to a vanishing contact force between the rollers and the borehole wall and thereby to a vanishing thrust force.
Furthermore, the rotative body of the known tool is directly connected to a drill bit provided at the elongate body, so that during operation the reactive torque from the drill bit is enhanced by the reactive torque from the rotative body.